Transverse Electromagnetic cells are often used to measure radiated EM emissions from electronics devices. Certain agencies, such as the Federal Communications Commission (FCC), require the measurement of EM emissions in order to guarantee that the electronic devices are not radiating EM emissions above limits specified by the agencies. These limits are based upon open area test site (OATS) measurement standards. Because a TEM cell can only measure EM emissions in a direction perpendicular to its longitudinal axis, a device under test must be repositioned during testing in such a way as to measure three mutually orthogonal vector components of these emissions. This repositioning involves rotating and tilting the device to change its orientation relative to the cell's longitudinal axis. Once the three sets of measurements are complete, they may be mathematically correlated for comparison to the OATS standard.
Because the longitudinal axis of most TEM cells is horizontal, or parallel to the ground, one of the three orthogonal components can be simply measured. Measurement of the two additional orthogonal components, however, requires rotating a device under test 90 degrees in two planes, which includes reorientation of the device from a horizontal to a vertical position wherein the device's orientation changes in relationship to the gravity vector. Some devices cannot be titled for measurement in a TEM cell without damaging the device or significantly modifying its operating characteristics.
There is a need, therefore, for a TEM cell that can measure three orthogonal components of EM emissions without requiring tilting of an electronic device from a normal, horizontal position. The testing apparatus of the present invention, described in greater detail below, provides a means for measuring three orthogonal components of EM emissions from an electronic device without requiring such realignment.